The invention relates generally to measurement of mechanical accelerations and more specifically to force balance accelerometers.
A conventional geophone involves a single coil and a magnet, both of which are contained in a housing. Springs support the coil, thus allowing motion in one dimension relative to the housing. The magnet is fixed with respect to the housing. The relative motion of the coil, in the magnetic field of the magnet, induces an electromotive force (“emf”), or voltage, in the coil. The coil is often a solenoid of finite length, in which a copper wire is wound circumferentially around a hollow, cylindrical, bobbin-form. By “single-coil,” it is meant that a single, continuous length of wire has been wound into a coil, and that connection to the coil is available only at the two ends of the wire. Specifically, in a single-coil design, there is no access to electrical signals at any intermediate point within the coil. FIG. 1 of U.S. Pat. No. 5,119,345, the disclosure of which is incorporated by reference, provides a cutaway drawing of a typical, conventional, single-coil, velocity geophone. The voltage across these two terminals provides the output signal from the device. This voltage is proportional to the velocity of the housing for frequencies above the natural resonant frequency of the spring-coil-mass system. Because of this property, the conventional geophone is often referred to as a velocity geophone.
Many applications, however, require direct sensing of acceleration, rather than velocity. Seismic techniques for imaging the subsurface structure of the earth can generate improved data if sensors measure acceleration directly. To this end, methods for converting the conventional single-coil velocity geophone to an acceleration-sensitive transducer are important.
Heretofore, several electronic techniques have been proposed for converting a conventional, single-coil, velocity geophone to a force-balance accelerometer. These methods utilize the ability of a conventional geophone's coil and magnet to simultaneously function as a force actuator and as a relative velocity sensor. In this case, a force is applied to the coil in response to an electrical current passing through it. This force actuator can be used to counteract or balance the force applied to the coil by external accelerations of the geophone housing. In this case, the resultant relative velocity of the geophone coil with respect to the geophone housing can be reduced to near zero. In such a force-balance arrangement, the force applied by the coil-magnet actuator is controlled in such a way as to reduce the emf from the same coil-magnet velocity to near zero. When balanced in this way, the force applied by the coil-magnet actuator is proportional to the external accelerations applied to the geophone housing. In that the applied force is proportional to the current flowing through the coil, measurement of this current provides a signal also proportional to the acceleration applied to the geophone housing. The technique is known generally as force-balance feedback.
In such a technique, electronic feedback is applied to effectively terminate the coil of a conventional geophone with a negative terminating impedance. The effect of this is that a current is induced in the geophone coil in response to external accelerations. This current applies a force that counteracts or balances the force applied to the coil by the external accelerations. The result is a force-balance sensor in which the current flowing through the terminating impedance is proportional to the current flowing through the geophone coil, and hence, proportional to the external acceleration applied to the geophone.